1. Field of the Invention
The present invention relates to a photopolymerizable composition. More particularly, the present invention relates to a photopolymerizable composition comprising a polymerizable compound having an ethylenically unsaturated bond, a photopolymerization initiator having a new composition and, if necessary, a linear organic polymer. The photopolymerizable composition is useful as a starting material for, for example, a photosensitive layer of a photosensitive printing plate sensitive to even light from a light source of an argon laser.
2. Prior Art
A process for duplicating an image by a photographic method with a photosensitive composition comprising a polymerizable compound having an ethylenically unsaturated bond, a photopolymerization initiator and, if necessary, a binder having a suitable film-forming capacity and a thermal polymerization inhibitor is well known. As described in U.S. Pat. Nos. 2,927,022, 2,902,356 and 3,870,524, such a photosensitive composition is photopolymerized and, therefore, cured and insolubilized when irradiated with light. Accordingly, a desired image can be formed with the photopolymerizable composition by forming a suitable film of the photosensitive composition, irradiating it with light through a negative of an intended image and removing only an unexposed part with a suitable solvent (this process will be hereinafter referred to as "development"). It is a matter of course that a photosensitive composition of this type is quite useful as a material for printing plates.
It has been known that when a polymerizable compound having an ethylenically unsaturated bond is used solely, a sufficient photosensitivity cannot be obtained. It has been proposed, therefore, to use a photopolymerization initiator for increasing the photosensitivity. Such photopolymerization initiators include, for example, benzil, benzoin, benzoin ethyl ether, Michler's ketone, anthraquinone, acridine, phenazine, benzophenone and 2-ethylanthraquinone. However, when such a photopolymerization initiator is used, it takes a long time to conduct the exposure for forming the image, because the photopolymerizable composition has only a low curing sensitivity. Therefore, in the reproduction of a minute image, it cannot be finely reproduced when even slight vibration takes place during the operation, and an increase in irradiation energy from the light source is necessitated in the exposing step. Consequently, the release of a large quantity of heat thus generated must be taken into consideration. Another defect is that the film made of the composition is deformed and denatured by heat.
The photopolymerization-initiating capacities of these photopolymerization initiators in the visible ray region longer than 400 nm are far lower than those in the ultraviolet region shorter than 400 nm. Therefore, the application of the photopolymerizable compositions containing a conventional photopolymerization initiator have been very limited.
Methods of improving a photopolymerizable system sensitive to the visible rays have been proposed. For example, U.S. Pat. No. 2,850,445 discloses that some kinds of photoreduction type dyes such as Rose Bengal, Eosine and Erythrocin have an effective sensitivity to the visible rays. Alternative techniques have been also proposed. In these techniques, a dye/amine complex initiator is used (Japanese Patent Publication for Opposition Purpose (hereinafater referred to as "J. P. KOKOKU") No. 44-20189); a combination system of a hexaarylbiimidazole with a radical-forming agent and a dye is used (J. P. KOKOKU No. 45-37377), a combination system of a hexaarylbiimidazole with a p-dialkylaminobenzylidene ketone is used (Japanese Patent Unexamined Published Application (hereinafter referred to as "J. P. KOKAI") Nos. 47-2528 and 54-155292); a combination system of a 3-keto-substituted coumarin compound with an active halogen compound is used (J. P. KOKAI No. 58-15503); and a combination system of a substituted triazine with a merocyanine pigment is used (J. P. KOKAI No. 54-151024). Although these techniques are effective for improving sensitivity to the visible rays, the sensitization velocities are still unsatisfactory. Under these circumstances, further improvement of the technique has been intensely demanded.
Now, investigation has been made in order to improve the sensitivity to U. V. rays and to form an image by means of laser. A U. V. projection exposure method for forming a printing plate, direct laser-engraving method, laser facsimile and holography are already practically in use. Further highly sensitive materials usable for these techniques are now being developed. However, their sensitivities are yet insufficient.